The present invention is related to corona chargers, and more particularly, to shielding of high voltage cables between the high voltage source and the charger assembly.
Corona chargers generate high frequency emissions that need to be suppressed by the shielding of high voltage cables. This suppression may be required for compliance with applicable electromagnetic emissions regulations and/or system functional requirements. In some applications a fully shielded solution is not required. A simple return path coaxial with the high voltage wire between the power source and the corona charger is all that is necessary. If the shield is placed close to the conductor there will be corona emissions and/or capacitive current through the high voltage wire insulation. This current loss through the insulation is typically in the range of tens to hundreds of microamps. In many electrophotographic charging applications current regulated power supplies are used to drive the charging systems. Typical current regulated chargers operate in the range of 100-2000 microamps. The loss of small amounts of current through unintended paths, such as through the shield, reduces the effectiveness of the system. Corona current flow from wire to shield reduces the efficiency of the system and degrades the insulation system resulting in lower reliability.
Electrophotographic printing machines are built in a variety of physical configurations requiring many different high voltage cable routings. It is desirable to have a shielding system that can easily be adapted to different lengths and shapes. A simple flexible shielding system that can provide a consistent minimum space between the high voltage conductors and the shield is required.
U.S. Pat. No. 3,758,700 issued to Ditscheid (hereinafter Ditscheid) describes a system for spacing the conductors of a coaxial power cable using flanges or ribs to position the inner conductor coaxially from the outer conductor. The ribs or flanges are used to position the inner conductor coaxially with respect to the outer conductor while providing adequate spacing to prevent corona discharge or arcing and providing means for heat dissipation. The teachings of Ditscheid, address high frequency and high power applications over significant lengths and employs multiple spacer components to provide the necessary spacing. Ditscheid provides a method for eliminating stiff axial insulation, which causes cables to be inflexible, by added multiple radial spacers integral to the cable and shield assembly. However, the teachings of Ditscheid do not present a simple method for adding a spaced shield to existing common low power high voltage wiring. In practice, electrophotographic imaging systems will have the location of corona chargers and their high voltage power sources vary. In some cases mounting and cabling can be done such that electromagnetic radiation is not a concern. In other applications high voltage cables require a ground return, between the power source and load, coaxial to the high voltage conductor. It is desirable to have a shielding system that can be adapted to various applications without the use of special cabling as described by Ditscheid.
U.S. Pat. No. 4,427,256 issued to Reif, et al. (hereinafter Reif) describes a system to prevent discontinuities at the terminations of a tightly spaced coaxial cable from enhancing the possibility of corona discharge between the inner conductor and the edges of the outer conductor. Reif recognizes the problems associated with termination of cable shielding in close proximity to the high voltage conductor. Reif has the shortcoming in that it does not address current loss through the corona shield on the cable. The teachings of Reif address the cable interconnects. The flow of corona and AC capacitive current from the center conductor to the shield along the length of the cable is not addressed. In low power, low current applications these losses can have a detrimental impact on the function of the system. The present invention addresses this problem within the prior art.
U.S. Pat. No. 6,225,565 issued to Prysner (hereinafter Prysner) describes a flexible conductive elastomeric material forming a Faraday shield that is used for EMI isolation. Prysner teaches the use of conductive particles within the elastomer that results in elastomeric material that shields, rather than acting as a spacer between the inner conductor and the shield. The EMI isolation technique taught by Prysner has the shortcoming of requiring numerous layers to construct and is, therefore, expensive to manufacture.
From the foregoing discussion, it should be readily apparent that there remains a need within the art for a continuous flexible spacer that can be used between high voltage conductors and shields that is capable of alleviating the problems associated with corona discharge through the insulation to the shield.
The present invention addresses the shortcomings within the prior art by providing a continuous flexible spacer between the high voltage conductor and the shield. The problems associated with corona discharge through the insulation to the shield can be eliminated while providing a low part count, and a simple to assemble package.
It is an object of the present invention to provide a simple process to create a flexible assembly by adding a spaced shield to existing common low power, high voltage wiring.
It is a further object of the present invention to eliminate loses by providing a spacer between the center conductor and the shield along the entire length of the shield, thus eliminating problems at the terminations.
The foregoing objects are provided by the invention in a corona charger with a high voltage shield having a high voltage conductor to transmit energy from the power source to the corona charger, a high frequency voltage shield surrounding the high voltage conductor, and a spacer between the shield and the conductor.
The invention disclosed provides an economical and adaptable system for providing EMI shielding on high voltage interconnection cables in corona charging systems. The shield can be implemented without loss of efficiency in the transmission of low-level currents from the power source to the charging device.